Diseases caused by one or more viruses are becoming alarmingly prevalent. Sexually transmitted viral infections and those transmitted by blood transfusion have been the focus of intensive research efforts over the past several years. Less severe diseases caused by viruses include the common cold.
Few truly effective antiviral agents have been developed, and in many cases the only treatment available is antibacterial agents, not to combat the virus but rather to ward off invasion of a weakened biological system by bacteria. The search continues for effective antiviral agents to treat and control common ailments such as influenza and colds, diseases caused by the various strains of herpes virus as well as to combat more severe viral infections such as acquired immune deficiency syndrome (AIDS).
This invention concerns the discovery that certain 2-amino-1,3,4-thiadiazole derivatives are useful in treating certain types of viral infections. Thiadiazoles are in general well known in the art. Some have been employed in the medical field. For example, Naik et al. describe the antimicrobial activities of a number of thiadiazolylthiazolidinones; J. Indian Chem. Soc., Vol. LX, July 1983, pp 674-678. Grant et al., in J. Med. Chem., Vol. 15, No. 10, 1972, pp 1082-1084, disclose that certain amine substituted derivatives of 2-amino-1,3,4-thiadiazole are useful hypotensive agents. U.S. Pat. No. 3,772,316 describes a class of N-acylated 2-amino-1,3,4-thiadiazoles which are said to be useful as fungicides. Russo et al., in Farmaco. Ed. Sci., Vol. 30, No. 12, 1975, pp 1031-1038, describe a series of thiourea derivatives of 1,3,4-thiadiazole and their cyclization products. The compounds are said to be antibacterial agents. Malinoski et al., in Virology, 110, 281-291 (1981) describes certain antiviral activity of 2-amino-1,3,4-thiadiazole.
In one embodiment, this invention provides a method for preventing, treating and controlling certain types of viral infections. More particularly one embodiment provides an in vitro antiviral method for protecting mammalian cells in culture which comprises adding to the culture an antiviral amount of a compound of formula (I): ##STR2## wherein: R.sup.1 is hydrogen, and R.sup.2 is cyano or --C(.dbd.S)--NH--R.sup.3 ; or
R.sup.1 and R.sup.2 taken together are .dbd.C(NH.sub.2).sub.2 or .dbd.C(R.sup.5)(NHR.sup.3 '); PA0 R.sup.3 and R.sup.3 ' are hydrogen or --COOR.sup.4 ; PA0 R.sup.4 is C.sub.1 -C.sub.10 alkyl, substituted C.sub.1 -C.sub.10 alkyl, C.sub.2 -C.sub.10 alkenyl, substituted C.sub.2 -C.sub.10 alkenyl, phenyl, or substituted phenyl; and PA0 R.sup.5 is hydrogen or --S--R.sup.6 where R.sup.6 is C.sub.1 -C.sub.4 alkyl, cyano-C.sub.1 -C.sub.4 alkyl or pyridyl-C.sub.1 -C.sub.4 alkyl.
The compounds of formula (I) are also useful in treatment of animals suffering from a viral infection or susceptible thereto. For this purpose, prefered compounds are those of the formulas (II), (III) and (IV): ##STR3## where R.sup.3 is hydrogen or --COOR.sup.4, R.sup.5 is hydrogen or --S--R.sup.6, and R.sup.6 is C.sub.1 -C.sub.4 alkyl.
The invention also provides a series of preferred new compounds defined by the formulas (V), (VI), and (VII): ##STR4## The compound of formula (V) is particularly useful as an in vivo antiviral. The compound of formula (VI) possesses both in vitro and in vivo antiviral activity, while the compound of formula (VII) is extremely active in vitro. The invention additionally embraces the use of pharmaceutically acceptable salts of the foregoing compounds.
Several of the thiadiazoles employed in the antiviral method of this invention are known in the art. All of the compounds can be prepared by chemical processes available, for example by the processes described by Naik et al. and Russo et al., supra.
Russo et al., supra., discloses 2-thioureido-1,3,4-thiadiazole, i.e. the compound of formula (III), as an antibacterial agent. No mention of antiviral activity is made.
R.sup.2 in the above formula (I) includes an acyl group defined by --C(.dbd.S)--NH--R.sup.3, where R.sup.3 can be another acyl moiety --COOR.sup.4. This definition of R.sup.2 provides the N-acyl thioureas of the invention, many of which are included in a preferred method of treatment. Such thioureas have the formula (II): ##STR5## where R.sup.4 has the above-defined meaning. Many of these thiadiazole thioureas are known in the art, for example as described in Japanese Pat. Nos. 71/35262, which is directed to mycocides, bacteriocides and herbicides, and 74/7218, which is directed to fungicides and bacteriocides.
In defining the foregoing compounds, R.sup.4 includes "C.sub.1 -C.sub.10 alkyl" and "substituted C.sub.1 -C.sub.10 alkyl". These terms refer to straight and branched chain alkyl groups such as ethyl, n-hexyl, isodecyl, 6-ethylheptyl, and optionally substituted alkyls such as haloalkyl, hydroxyalkyl, phenylalkyl and the like. "Halo" includes fluoro, chloro, bromo and iodo. R.sup.4 can additionally be C.sub.2 -C.sub.10 alkenyl or substituted alkenyl such as allyl, 4-hexenyl, 3-chloro-5-heptenyl, 2-hydroxy-4-isoheptenyl and the like, as well as substituted phenyl such as chlorophenyl, hydroxyphenyl, methylphenyl, cyanophenyl and the like. Preferred R.sup.4 groups are C.sub.1 -C.sub.6 alkyl and C.sub.2 -C.sub.6 alkenyl.
Most of the foregoing compounds are named herein as carbamimidothioic acid derivatives and can be prepared by reacting a 1,3,4-thiadiazol-2-ylthiourea with an alkylating agent of the formula R.sup.6 X where R.sup.6 is as defined above and X is a leaving group. Suitable leaving groups include Cl, Br, I, and sulfonic ester groups like tosylate and mesylate. Thus, suitable alkylating agents include, for example, an alkyl halide such as methyl bromide, or a substituted alkyl halide such as cyanomethyl bromide or 2-pyridylmethyl iodide. The reactions are typically accomplished by mixing approximately equimolar quantities of a 2-thiadiazolyl thiourea and an alkylating agent in the presence of a base such as sodium carbonate and in an unreactive organic solvent such as dimethylformamide of acetonitrile. The reaction is normally complete within about sixteen hours when carried out at about 20.degree. C. to about 50.degree. C. The reaction solvent can be removed by evaporation under reduced pressure if desired. The product is readily purified by routine methods, including crystallization from solvents such as ethanol, ethyl acetate, hexane; or chromatography over solid supports such as silica and the like.
A particularly preferred group of compounds provided by this invention includes 1,3,4-thiadiazol-2-cyanamide and its pharmaceutically acceptable salts. The compound can be prepared by reacting a carbamimidothioic acid ester with an oxidizing agent such as metachloroperbenzoic acid, peracetic acid, sodium peroxide, hydrogen peroxide, ozone, chlorine or the like.
1,3,4-Thiadiazole-2-cyanamide can also be prepared by reacting a protected 2-amino-thiadiazole derivative, namely 4-imino-3-phenylmethyl-1,3,4-thiadiazole with cyanogen bromide and then removing the protecting group at the 3-position by debenzylation with a Lewis acid, e.g. aluminum chloride. The debenzylation is carried out in an organic solvent such as methylene chloride, toluene or benzene. Preferably at least 2 equivalents of Lewis acid are used, and more preferably 4 to 8 equivalents are used. The temperature is not critical. The debenzylation can be carried out, for example, at from 0.degree. C. to 90.degree. C. Room temperature is preferred.
The cyanamide is also produced when a carbamimidiothioic acid ester is reacted with ammonia, as illustrated in Example 8.
The cyanamide readily forms pharmaceutically acceptable salts by reaction with organic and inorganic bases such as sodium acetate, calcium carbonate, sodium hydroxide and the like.
The compounds of this invention can exist in several tautomeric forms, all of which are included within the scope of the invention. For example, a preferred compound, 1,3,4-thiadiazol-2-cyanamide, can exist as follows: ##STR6## Another example of tautomeric forms of an invention compound is represented by the following formulas: ##STR7## All possible tautomeric forms of the invention compounds are embraced herein.
The following detailed examples illustrate the synthesis of thiadiazoles to be employed in the antiviral method of this invention.